The present invention is directed to an optical flow cell
design that can be cleaned automatically without invasive physical means. It is
particularly useful when optical measurements are made following chromatographic
separation since these are often associated with particulate material and air bubbles
which tend to adhere to the optical surfaces themselves.

Background

In the field of light scattering, as applied to determine
the molar mass and mean square radius of solvated molecules, measurements are made
from solutions comprised of a solvent containing a dissolved sample. By measuring
the scattered light variation with scattering angle and measuring the concentration
of the solute, one may in principle determine the molar mass and mean square radius
of such solvated molecules. Similarly, the light scattering properties of sub micrometer
particles in liquid suspension may be used to determine their average size. Light
scattering techniques may be applied as well for measurements involving inelastic
light scattering such as photon correlation spectroscopy, Raman spectroscopy, fluorescence,
etc. These measurements, usually performed at a fixed single angle, are used to
determine the hydrodynamic size of the particles or molecules illuminated.

Light scattering measurements are often made with a light
scattering photometer wherein the sample is introduced into an optical cell such
as shown in US-A-4,616,927 and US-A-5,404,217. Interfering with such optical measurements are a variety of contaminants
whose presence inside the flow cell often contribute to the recorded light scattering
signals in such a manner as to distort or even mask them. Such contaminants arise
from various sources, many of which cannot be avoided. Included among these are
small air bubbles, fine particles shedding from chromatographic columns if such
are being employed to separate the molecules or sub-micrometer particles prior to
measurement, aggregates formed from the sample itself which may have a strong affinity
for the internal optical surfaces, contaminants in the poorly prepared solvent,
debris from previous measurements that build up on the optical surfaces, etc.

During the measurement process, the presence of these contaminants
is often recognized indirectly through the effects they have on the scattering or
are noticeably visible through physical examination of the scattering cell, or both.
There are various means by which such contaminants are removed or dislodged from
the internal optical surfaces such as flushing the optical cell with different solvents
such as acids or detergents or introducing a large air bubble in the manner of the
familiar Technicon AutoAnalyzer of the 1960s. Sometimes, no matter how much effort
has been expended, the flow cell must be disassembled and each component cleaned
manually. Once disassembled, one of the most useful means for cleaning surfaces
is to use ultrasonic waves as created in an ultrasonic cleaning bath. The components
are placed in a fluid such as water and ultrasonic waves whose fixed frequencies
are of the order 50 kHz are propagated throughout the bath. These waves are generally
generated by means of piezoelectric transducers well coupled to the bath chamber.
At the frequencies and power levels traditionally applied, cavitation effects generally
cause the generation of bubbles which, when driven against a surface, tend to assist
in the cleaning and scrubbing of such surfaces.

Although the disassembly of an optical cell and the subsequent
cleaning of its parts in an ultrasonic bath are effective, it is time consuming.
Unfortunately, it is often the only means possible. When the optical cell is used
in a high temperature environment, such as is the case for chromatographic separations
requiring high temperature solvents, the traditional disassembly concept becomes
even more time-consuming since the temperature of the chromatograph itself must
often be reduced significantly to obtain access to the optical cell which is then
removed and cleaned. High temperature chromatographs, and especially the columns
used therein, can be damaged during temperature cycling which, therefore, must be
carefully executed. The process of cleaning an internally mounted optical cell can,
in such a case, require up to 24 hours to effect a removal, cleaning, and reinstallation.

It always has been thought desirable to have optical elements
of the light scattering cells designed in such a manner as to prevent the deposition
of extraneous materials on their surfaces or, at the very least, design them in
such a manner as to permit the cleaning of their internal surfaces with minimal
effort. To this end, many structures requiring clean, particulate-free surfaces
have been designated as "self-cleaning" such that once internal precipitants are
detected they may be removed without need for disassembling the structures themselves.
A process by which the initial formation of such contaminants may be reduced is
taught, for example, by US-A-5,442,437 wherein windows, through which optical measurements are to be made, are
so positioned that they extend into the flowing solution which, thereby, continuously
"...scour said window to minimize contamination and clouding [thereof]...." This,
of course, is an old concept that was disclosed in US-A-4,616,927, referenced above, and numerous other similar implementations whereby
it is necessary to clean observation windows of various types. Although such cleaning
may keep the observation windows clear of particulate debris for some time, eventually
sufficient particles may accrete so as to interfere with light passing through some
optical surface.

Another example of a self cleaning cell is US-A-4,874,243 wherein the windows are at an angle to the direction of flow which results
in a "...self cleaning action..." as the flowing stream passes over them. A similar
example is US-A-4,330,206 wherein is shown a measurement chamber "...inherently self-clearing of
air or gas bubbles in liquid samples... [which provide] inherently efficient cleansing
of the measurement chamber...." This is achieved by outlet means lying above the
optical region guiding thereby air bubbles up and out of the fluid enclosed. The
fluid flowing into the measurement channel strikes the cell window obliquely, thus
cleaning it and maintaining it free of contaminants.

US-A-4,496,454 describes another example of a self-cleaning mechanism for the case of
electrochemical cells used with certain forms of liquid chromatography. This prior
art attacks a similar problem for electrochemical detection that faces light scattering
detection: the fouling of the electrode surfaces during measurement which, in turn,
affects the detector response. In the light scattering case, the optical surfaces
can become fouled with particulates and small air bubbles. This prior art achieves
the cleaning by using a capillary tube to generate a water jet perpendicular to
the detector electrode surface

In addition to such fluid cleaning means described above,
there exist a number of mechanical means exemplified by US-A-5,185,531. In this implementation, the optical windows are kept clean by introducing
periodically mechanically controlled flexible wiper blades extending from opposite
sides of [a]... blade holder for wiping engagement with the window surfaces...."
Other implementations of a wiping motion to clean an optical cell may be found in
US-A-3,844,661 or US-A-4,074,217.

Although the use of ultrasonic waves appears an attractive
means for removing particulates from surfaces, such as described in US-A-4,457,880, it has never been used as a component of an optical cell to permit self
cleaning action. There are three basic reasons for this omission. First is the fact
that there has been neither means for establishing a proper frequency regime to
achieve such cleaning nor means for localizing the cleaning action to the internal
cell surfaces that require it. Secondly, even were such a self cleaning device integrated
with the cell structure, there could be no assurance that, once removed from the
internal cell surfaces, the particulates would not re-adhere or simply remain within
the cell to adhere later to some other region. Finally, traditional ultrasonic waves
used in cleaning are generated at frequencies of the order or 50 kHz, which, at
the power levels traditionally employed and in fluids such as water, induce cavitation
effects that result in the generation of bubbles. Such bubbles are most helpful
because of their implied scrubbing action on the surfaces to be cleaned. Were such
bubbles generated within an optical cell, the bubbles themselves could be expected
to adhere to surfaces within the fine interstices of such cells defeating, thereby,
the cleaning concept ab initio.

The teaching of US-A-4,672,984 extends the ultrasonic concept for cleaning optical surfaces by providing
a plurality of cleaning steps, each of which may involve a different working liquid
and/or ultrasonic intensity applied over varying periods of time. Once again, such
cleaning is done externally to any enclosed structure with the parts to be cleaned
transported individually to the array of cleaning baths. This prior art does not
discuss the frequency of the applied ultrasonic frequencies nor any possible variations
thereof, so one assumes that the standard cavitation prone frequencies around 50
kHz is, employed.

US-A-5,656,095 introduces the concept of multiple frequencies, some of which are applied
intermittently to destroy the bubbles generated by the continuously applied frequency.
Such an action results in corresponding pressure pulses to which is attributed a
"...greatly improved..." washing effect. This prior art considers so-called low
frequency generation as occurring at frequencies of 28 kHz, 45 kHz, and 100 kHz
whereas high frequency generation describes generation at 160 kHz. The high frequency
ultrasonic waves are said to generate bubbles in the size range of 20µm to
500µm while the intermittent low frequency waves destroy the bubbles, generating
as they collapse, even higher orders of ultrasonic waves.

US-A-5,889,209 discloses a method and an apparatus for preventing biofouling of aquatic
sensors. In this prior art, a transducer which is driven by an ultrasonic generator
for emitting ultrasonic pressure waves is placed in an aquatic environment. To protect
the transducer against damage from the aquatic environment, it is housed in an enclosure
which separates the transducer from the aquatic environment. The transducer and
the enclosure form a submersible ultrasonic assembly which is positioned in proximity
to, but spaced from a membrane covered probe surface of a dissolved oxygen sensor.
The ultrasonic pressure waves emitted from the transducer assembly interact with
the probe surface of the sensor to prevent biofouling, wherein the probe surface
comprises the surface of the membrane placed over an electrode of the sensor. The
distance between the probe surface and the enclosure wall to which the transducer
is attached can be varied depending on the particulars of the transducer and the
ultrasonic generator. With respect thereto, in this document it is given that "a
typical separation is on the order of 4 mm to 10 mm for effective ultrasonification
of the probe surface". Further, in this prior art, examples of the mentioned aquatic
environment are given as ocean, lake, river, wastewater treatment basin, aqua culture
tank, laboratory container or biological reactor.

CH-A-636 200, representing the closest prior art from which the present invention proceeds,
discloses a method and a device for removing gas bubbles and other precipitations
from cuvettes. This prior art teaches the provision of a flow cell with all associated
support and mounting elements and means for attaching an ultrasonic wave generator
in firm mechanical contact with the flow cell.

The present invention is concerned with the implementation
of an ultrasonic cleaning device that is integrated with an optical flow cell and
controlled in such a manner as to permit sonic coupling with those internal regions
of the cell most needed to be particulate free. Sonic waves are used in a manner
by which cavitation is avoided whenever possible since such cavitation can cause
etching or other damage to finely polished optical surfaces.

Summary of the invention

According to a first aspect of the present invention, there
is provided a method for cleaning an optical flow cell device containing optical
elements through whose surfaces light must pass, comprising the steps of

a) providing means by which an ultrasonic wave generator is attached in firm
mechanical contact with said flow cell;

b) selecting a variable range of ultrasonic frequencies best coupled to the
internal regions of said flow cell where affixed particulates may occur for purposes
of dislodging particulates;

c) driving said ultrasonic wave generator means over said range of ultrasonic
frequencies selected; and

d) flowing a particulate free fluid through said flow cell during the period
when said attached ultrasonic wave generator is activated to generate frequencies
over the range being scanned.

According to a second aspect of the present invention,
there is provided an optical flow cell device capable of self cleaning, comprising
a flow cell with all associated support and mounting elements, and a means for attaching
in firm mechanical contact with said flow cell an ultrasonic wave generator, characterized
by a means for driving said ultrasonic generator over a variable range of ultrasonic
frequencies, and means to flow through said optical flow cell a source of particulate
and bubble free fluid throughout the period in which said ultrasonic generator is
activated.

Further advantageous embodiments are defined in the dependent
claims.

This invention presents a new design concept for the cleaning
of optical surfaces within flow cells used in conjunction with light scattering
measurements such as commonly employed in the field of analytical chemistry and,
more particularly, for liquid chromatography. Basic to this invention is the incorporation
into the flow cell structure itself of means to provide internal to the flow cell
extremely high frequency sonic waves such as would be produced by means of an electrically
driven piezoelectric transducer. The frequencies of these waves are much greater
than those employed according to US-A-5,656,095. In order to avoid cavitation, yet be in resonance with the typical internal
dimensions of the cleansed flow cells, frequencies of the order of 1 MHz are employed.
There are many different types of flow cells for which this design would be useful
including those referenced above. In B. Chu's textbook on "Laser light scattering",
a number of additional designs may be found; though these are by no means exhaustive.

Key to this invention are four features: 1) integrating,
by good mechanical contact means, the sonic source, a piezoelectric transducer in
the preferred embodiment, and the optical flow cell; 2) varying the frequency of
the applied ultrasonic waves so as to couple well with those internal regions where
the dislodgment of particulates is required; 3) using frequencies of the order of
1 MHz which are much greater than those traditionally used for ultrasonic cleaning
purposes and, at practical power levels, beyond the frequencies that conventionally
would cause cavitation in most liquids; and 4) providing a flowing fluid means during
the application of the ultrasonic waves by which dislodged particulates may be removed
from the cell.

Although such an integrated cleaning technique may be applied
to static optical cells that are not generally operated in a flow through mode,
when used with such mechanically coupled ultrasonic waves, means must be provided
to permit a flow stream to remove particles dislodged by said sonic cleaning during
the application of said ultrasonic waves.

The requirement that the frequency of the applied sonic
waves must be adjustable, so as to couple the sonic energy most efficiently to the
internal regions of the flow cell structure most prone to the presence of unwanted
particulates, may equally well be served by automatically, and repetitively, scanning
a range of frequencies that includes those best suited for the internal regions
to be cleaned. Note that at frequencies of the order of 1 MHz in water, the associated
wavelengths are of the order of 1.5 mm, approximately the diameter of the flow cell
of US-A-4,616,927 and US-A-5,404,217 and related structures. The dislodgment of particles by the present inventive
means relies upon mechanical displacement by the ultrasonic waves themselves rather
than the more traditional scrubbing action created in large measure by the cavitation
created air bubbles turbulently bombarding the affected surfaces.

The fluid that must be flowing through the flow cell structure
during application of the ultrasonic waves throughout the structure must be in itself
particle-free. When applied to a flow cell in conjunction with a chromatographic
separation, this fluid would correspond to the so-called mobile phase of the chromatographic
separation process. Such fluids should be free of particulates and are often degassed
and filtered prior to use in the chromatograph. Additionally, since the ultrasonic
field can induce particle aggregation within the flow cell, the resulting aggregates
are more easily flushed from the flow cell.

Brief description of the drawings

Figure 1 is an exploded view of a flow cell similar to the type disclosed in
US-A-5,404,217.

Figure 2 shows a top view of the cell of Fig. 1 providing a port through which
may be observed the illuminating laser beam and bore.

Figure 3 shows a top view of a piezoelectric transducer coupled to a flow cell
structure, which permits direct observation of particulates therein.

Detailed description of the invention

Figure 1 shows an exploded view of the key elements of
a flow cell of the type disclosed in US-A-5,404,217. A manifold comprised of elements 1, 2, and 3 hold a glass
cell 4 through which is a bore 5. At each manifold end is a glass
window 6, 7 suitably sealed by O-ring means 8 and locking fixtures
9. Fluid, containing solvated molecules or entrained particles enters through
fitting 10 and exits cell through 11. An illumination source, usually
a focused beam 12 from a laser 13, enters through window
6. This figure shows a characteristic flow cell structure containing many
internal surfaces and regions capable of trapping particulates or permitting precipitates
to form thereon. A top view of this cell is shown in Fig. 2 providing a port
14 through which the laser beam 12 and bore 5 may be observed.
In the event there are particles 15 present on the walls of the bore
5, often they may be visually observed appearing as bright sources of light.
Accordingly, it is an objective of this invention to provide a means by which such
extraneous light sources, arising from particulates affixed to the cell walls, may
be removed from this type of flow cell as well as any other structures wherein such
particulates may become affixed.

In the present specification, the term "flow cell" is used
to describe a structure comprised of the glass cell itself, the windows through
which the incident beam of light enters, and all supporting and ancillary elements
such as the various pieces of the manifold shown in Fig. 1. Although the light source
for the preferred embodiment of this invention is generally referred to as a laser,
the invention applies equally well to other types of optical flow cells where their
light source may be from incandescent lamps, light emitting diodes, arc lamps, etc.,
or even internally generated by constituents of the sample itself.

A preferred embodiment of the invention is shown in Fig.
3 wherein a piezoelectric transducer 16 is maintained in mechanical contact
with the flow cell 17 by illustrated means as follows: direct contact plate
18 to which said piezoelectric transducer 16 is bonded, electrically
conductive spring 19 compressed against said transducer by washer means
20 which distributes pressure evenly as imparted from spring washer
21, and threaded retainer 22 which hold assembly within housing module
23. The assembly housing 23 is mechanically attached to the read head
24, holding the flow cell 17, by bolt means 25. Power is supplied
to said transducer via power connector means 26.

The preferred embodiment, just described, provides for
firm mechanical contact with the flow cell of the contact plate 18 to which
the piezoelectric transducer 16 is attached. The mechanical contact is achieved
by pressure means imparted to piezoelectric transducer/contact plate via compression
of the spring washer 21 and conductive spring 19 by the compression
occurring as the threaded retainer 22 is threaded into the assembly housing
23. The housing 23 may include a stopping means whereby a limiting
compression may be set. Alternatively, said piezoelectric transducer may be attached
directly through bonding or other affixation means including gluing or cementing
using epoxies or other adhesives. Obviously, there are many other locations on any
given flow cell structure where such transducer device may be attached to make good
mechanical contact for use subsequently to generate sonic waves permeating throughout
said flow cell bore and other internal regions wherein particulates may form or
become attached.

The concept of attaching a piezoelectric transducer directly
to a surface for purposes of removing particulates is not new. For example,
US-A-5,724,186 shows how such an attachment of two piezoelectric transducers, in a so-called
bi-morph configuration, can provide a means for clearing a vehicular rear view mirror
of water droplets. However, the concept of attaching an ultrasonic transducer to
a structure for purposes of cleaning inaccessible internal enclosed regions is new
and unique. Note that the particles are limited to water droplets which must be
in an air environment with the mirror face essentially parallel to the earth's gravitational
field.

In the preferred embodiment of an electronic driving circuit
for powering the ultrasonic transducer, of the type exemplified by a piezoelectric
transducer, it should generate sonic waves spanning a broad swept range of frequencies.
Since it is not generally possible to predict exactly the frequency that would couple
best with the particular internal regions of the flow cell structure wherein affixed
particles would be loosened therefrom by the corresponding sonic waves, the preferred
embodiment of this invention allows for the sweeping of the excitation frequency
generated by the piezoelectric transducer. Each internal region will have an associated
range of frequencies best coupled for purposes of dislodging particulates. Therefore,
by sweeping the frequency applied, one can insure that such optimal drive frequencies
will have been applied. We have found that the swept range should be between about
0.5 MHz and 5 MHz for the structures such as shown in Fig. 1.

Although the preferred embodiment of this invention suggests
that extremely high intensity ultrasonic waves be employed operating at the megahertz
range so as to couple more effectively with the internal elements of the optical
flow cell, this is certainly not the first time that such frequencies have been
employed for cleaning purposes. The Branson Ultrasonics Corporation of Danbury CT,
for example, offers for sale its 400 kHz MicroCoustic ® device capable
of cleaning "... irregular geometries, tight clearances and highly finished surfaces..."
by non-cavitational means. However, there is no variation of frequency nor is the
device integrated with the object to be cleaned. It is representative of the traditional
immersion bath methods, though operated at a higher frequency. Again, all surfaces
to be cleaned are external surfaces, though the Branson concept emphasizes the cleaning
of surfaces containing very fine features. Accessibility of the ultrasonic waves
to these fine features requires that these surfaces be placed within baths providing
direct exposure to said ultrasonic waves. The possibility of coupling external sonic
sources to a structure whose inner surfaces contained fine features to be cleared
of adhered particles was never considered for possible application of the Branson
ultra high frequency devices. This is because the Branson devices and similar devices
manufactured by others are designed to clean by ultrasonic means a broad range of
parts which do not include parts and surfaces internally situated with respect to
complex structures such as optical flow cells.

The dislodgment of particles by the present inventive means
relies upon the mechanical displacement by the ultrasonic pressure waves themselves
rather than the more traditional scrubbing action created in large measure by the
cavitation-created air bubbles turbulently bombarding the surfaces to be cleaned
of particles. Cavitation induces the dissolution of gas from the fluid and this
can result in bubbles, which, like any other foreign particulates present in the
optical cell, are inimical to the performance of light scattering measurements where
they may interfere with the scattered or incident light. Note that at frequencies
of the order of 1 MHz in water, the associated ultrasonic wavelengths are of the
order of 1.5 mm, approximately the diameter of the flow cell of US-A-4,616,927 and US-A-5,404,217 and related structures. Such waves may propagate longitudinally and throughout
the flow channels producing pressure fluctuations both transverse and parallel to
the optical surfaces thereon. Operating the ultrasonic piezoelectric transducers
at conventional power levels and ultrasonic frequencies of the order of 50 kHz would
generally result in the creation of additional gas bubbles further contaminating
the flow cell and optics. However, at sufficiently low power levels for most fluids,
such cavitation effects could be minimized, at the expense of cleaning efficiency.
During experiments with the inventive concept, it has been noted that although the
sonic waves effectively dislodge the particulates from the optical regions within
typical flow cells, these same particulates are driven to other proximate regions
where they again become affixed. Particulates were seen also forming aggregates
with other particles; such aggregates being caused by the impressed ultrasonic fields.
This self-scavenging effect further helps collect dispersed particles as the applied
flow stream more easily drives out larger particulates because of their greater
cross section. In order to drive them out of the flow cell, it is essential that
a particle free flow be directed through the cell during the ultrasonic dislodgment
process. In this manner, the particulates are forced to progress toward the cell
outlet while executing a somewhat random walk from one region of the cell surface
to another. Even in the presence of such an imposed flow, particles are often observed
to move against the stream and become re-affixed up stream. However, these are but
statistically random motions, which are then superimposed upon the steady stream
flow resulting in their eventual removal from the flow cell. The total time required
to clear the cell of Fig. 1, for example, is of the order of a minute. Thus it is
not necessary that the impressed sonic cleansing action be always functioning. Its
activation is, therefore, generally controlled by the operator of the light scattering
apparatus on the basis of his/her observation of the light scattering signals being
collected. Naturally, such periodic cleaning could be programmed to occur automatically
using such light scattering signals and establishing therefrom the criteria indicative
of the presence of particulate contaminants.

The imposed fluid flow through the flow cell structure
during application of the ultrasonic waves throughout the structure must be in itself
particle-free. When applied to a flow cell used for making light scattering measurements
following chromatographic separation, this fluid would correspond to the so-called
mobile phase. Such fluids should be free of particulates and are often degassed
and filtered prior to use in the chromatograph.

For various types of optical cells wherein static or dynamic
light scattering measurements are to be made and there is no other source of continuously
flowing fluid to perform such flushing, it may necessary to attach or otherwise
provide means by which such fluids may be introduced and removed from such cells
in a continuous manner to carry out of said optical cells particles dislodged by
the applied ultrasonic waves. This fluid itself, of course, must be free of particles
and this usually requires both prefiltering and degassing.

The application of the present invention for optical cells
that are used within chromatographs at elevated temperatures is a particularly important
one. As has been discussed earlier, the traditional disassembly and cleaning procedures
become even more time-consuming since the temperature of the chromatograph itself
must often be reduced significantly to obtain access to the optical cell which is
then removed and cleaned. High temperature chromatographs, and especially the columns
used therein, can be damaged during temperature cycling which, therefore, must be
carefully executed. The process of cleaning an internally mounted optical cell can,
in such a case, require up to 24 hours to effect a removal, cleaning, and reinstallation.
The incorporation of the self-cleaning structure in such high temperature chromatographs
is, therefore, both desirable and essential. The preferred embodiment of the invention
using a piezoelectric ultrasonic generator should be capable of operation at temperatures
as high as 250 °C.

A further problem that must be considered when such an
implementation of the invention is employed concerns the ever-present fire dangers
when organic solvents are used at both ambient and high temperatures. Since the
ultrasonic circuitry requires application of voltages of the order of 100 V, there
will exist the possibility of a spark-initiated discharge. Accordingly, for such
cases, it is important that a vapor detector (such as manufactured by Figaro USA,
Inc.) be present in close proximity to the ultrasonic transducer. The vapor detector
can itself be used as a safety interlock to prevent operation of the transducer
whenever such a leak poses a fire or explosion danger.

A method for cleaning an optical flow cell device containing optical
elements through whose surfaces light must pass, comprising the steps of
a) providing means (19, 21, 22) by which an ultrasonic wave generator
(16) is attached in firm mechanical contact with said flow cell (17);b) selecting a variable range of ultrasonic frequencies best coupled
to the internal regions of said flow cell (17) where affixed particulates may occur
for purposes of dislodging particulates,c) driving said ultrasonic wave generator means (16) over said range
of ultrasonic frequencies selected; andd) flowing a particulate free fluid through said flow cell (17) during
the period when said attached ultrasonic wave generator (16) is activated to generate
frequencies over the range being scanned.The method of claim 1 wherein said ultrasonic generator (16) is a piezoelectric
transducer. The method of Claim 1 wherein said range of ultrasonic frequencies selected
is between 0.5 and 5 MHz.The method of Claim 1 wherein said optical flow cell device is provided
as the flow cell component of a light scattering photometer.The method of Claim 4 wherein said light scattering photometer is used
in combination with a liquid chromatograph.The method of Claim 1 wherein said particulate free fluid is the mobile
phase used with a liquid chromatographic separation.The method of Claim 1 wherein said firm mechanical contact is achieved
by adhesive means.The method of Claim 7 wherein an epoxy material is used as said adhesive
means.The method of Claim 1 wherein said firm mechanical contact is provided
by spring pressure means.An optical flow cell device capable of self cleaning, comprising
a flow cell (17) with all associated support and mounting elements (1-4, 23,24);
and
a means (19, 21,22) for attaching in firm mechanical contact with said flow cell
(17) an ultrasonic wave generator (16);
characterized by
a means for driving said ultrasonic generator (16) over a variable range of ultrasonic
frequencies; and
means to flow through said optical flow cell (17) a source of particulate and bubble
free fluid throughout the period in which said ultrasonic generator (16) is activated.The device of Claim 10 wherein said ultrasonic generator (16) is a piezoelectric
transducer.The device of Claim 10 wherein said variable range of frequencies is
between 0.5 and 5 MHz. The device of Claim 10 being the flow cell component of a light scattering
photometer.The device of Claim 13 wherein said light scattering photometer is provided
in combination with a liquid chromatograph.The device of Claim 10 wherein said particulate free fluid is the mobile
phase used with a liquid chromatographic separation.The device of Claim 10 wherein said firm mechanical contact is provided
by spring pressure means.The device of Claim 10 wherein said firm mechanical contact is achieved
by adhesive means.The device of Claim 17 wherein said adhesive means is an epoxy material.